Determining IP Routes

Introducing Distance Vector Routing

Outline

OverviewDistance Vector Route SelectionRouting Information MaintenanceRouting Inconsistencies with Distance Vector Routing ProtocolsCount to Infinity PreventionTechniques to Eliminate Routing LoopsImplementation of Techniques to Eliminate Routing LoopsSummary

Slide 1 of 2

Purpose: This slide states the chapter objectives.

Emphasize: Read or state each objective so that each student has a clear understanding of the chapter objectives.

Note: Catalyst switches have different CLIs. The Catalyst 2900xl and the Catalyst 1900 has a Cisco IOS CLI. The Cisco IOS CLI commands available on the 2900xl is different from the 1900. The Catalyst 5000 family has no Cisco IOS CLI, and use the set commands instead. This class only covers the configuration on the Catalyst 1900 switch.

Routers pass periodic copies of their routing table to neighboring
routers and accumulate distance vectors.

Distance Vector Routing Protocols

Purpose: This figure introduces the distance vector routing algorithm, the first of the classes of routing protocols, and outlines how it operates.

Emphasize: Distance vector algorithms do not allow a router to know the exact topology of an internetwork.

This information is somewhat analogous to the information found on signs at a highway intersection. A sign points toward a road leading away from the intersection and indicates the distance to the destination.

Further down the highway, another sign also points toward the destination, but now the distance to the destination is shorter.

As long as each successive point on the path shows that the distance to the destination is successively shorter, the traffic is following the best path.

Routers discover the best path to destinations from each neighbor.

Sources of Information and Discovering Routes

Layer 3 of 3

Emphasize: Layer 3 adds the final entries received some time later that have distances of 2 from routers A and C.

Selecting the
Best Route with Metrics

Emphasize: How the routing algorithm defines best determines the most important characteristics of each routing algorithm.

Hop countSome routing protocols use hop count as their metric. Hop count refers to the number of routers a packet must go through to reach a destination. The lower the hop count, the better the path. Path length is used to indicate the sum of the hops to a destination. As indicated in the figure, RIP uses hop count for its metric.

TicksMetric used with Novell IPX to reflect delay. Each tick is 1/18th of a second.

CostFactor used by some routing protocols to determine the best path to a destination; the lower the cost, the better the path. Path cost is the sum of the costs associated with each link to a destination.

BandwidthAlthough bandwidth is the rating of a links maximum throughput, routing through links with greater bandwidth does not always provide the best routes. For example, if a high-speed link is busy, sending a packet through a slower link might be faster. As indicated in the figure with highlighting, delay and bandwidth comprise the default metric for IGRP.

DelayDepends on many factors, including the bandwidth of network links, the length of queues at each router in the path, network congestion on links, and the physical distance to be traveled. A conglomeration of variables that change with internetwork conditions, delay is a common and useful metric. As indicated in the figure with highlighting, delay and bandwidth comprise the default metric for IGRP.

LoadDynamic factor can be based on a variety of measures, including CPU use and packets processed per second. Monitoring these parameters on a continual basis can itself be resource intensive.

Updates proceed step by step from router to router.

Maintaining Routing Information

Layer 3 of 3

Layer 3 adds router B, which receives the updated routing table from routerA. In turn, router B will perform its own process to update its routing table given this new topology update from router A.

Distance vector updates occur step by step.

Typically, a router sends updates by multicasting its table on each configured port, but other methods, such as sending the table only to preconfigured neighbors, are employed by some routing algorithms.

Multicast is used by the RIP2, OSPF, and EIGRP routing protocols. RIP and IGRP use broadcast.

The routing table can be sent routinely and periodically, or whenever a change in the topology is discovered. Updates sent when changes occur are called triggered updates.

Each node maintains the distance from itself to each possible destination network.

Inconsistent Routing Entries

Slide 1 of 4

Purpose: This figure describes the first of the general problems that a distance vector protocol could face without the corrective influence of some countermeasure.

Emphasize: Layer 1 shows the original state of the network and routing tables. All routers have consistent knowledge and correct routing tables. In this example, the cost function is hop count, so the cost of each link is 1. Router C is directly connected to network 10.4.0.0 with a distance of 0. Router As path to network 10.4.0.0 is through router B, with a hop count of 2.

Slow convergence produces inconsistent routing.

Inconsistent Routing Entries (Cont.)

Slide 2 of 4

Emphasize: In Layer 2, router C has detected the failure of network 10.4.0.0 and stops routing packets out its E0 interface. However, router A has not yet received notification of the failure and still believes it can access network 10.4.0.0 through router B. Router As routing table still reflects a path to network 10.4.0.0 with a distance of2.

Router C concludes that the best path to network 10.4.0.0 is through Router B.

Inconsistent Routing Entries (Cont.)

Slide 3 of 4

Emphasize: Because router Bs routing table indicates a path to network 10.4.0.0, router C believes it now has a viable path to 10.4.0.0 through router B. Router C updates its routing table to reflect a path to network 10.4.0.0 with a hop count of 2.

Router A updates its table to reflect the new but erroneous hop count.

Inconsistent Routing Entries (Cont.)

Slide 4 of 4

Emphasize: In Layer 4, router A receives the new routing table from router B, detects the modified distance vector to network 10.4.0.0, and recalculates its own distance vector to network 10.4.0.0 as 3.

If all routers in an internetwork do not have up-to-date, accurate information about the state of the internetwork, they might use incorrect routing information to make a routing decision.

The use of incorrect information might cause packets to take less-than-optimum paths or paths that return packets to routers that they have already visited.

The hop count for network 10.4.0.0 counts to infinity.

Count to Infinity

Purpose: This figure describes another of the general problems that a distance vector protocol could face without the corrective influence of some countermeasure.

Emphasize: Both routers conclude that the best path to network 10.4.0.0 is through each other and continue to bounce packets destined for network 10.4.0.0 between each other, incrementing the distance vector by 1 each time.

This condition, called count to infinity, continuously loops packets around the network, despite the fundamental fact that the destination network 10.4.0.0 is down. While the routers are counting to infinity, the invalid information allows a routing loop to exist.

A related concept is the Time-to-Live (TTL) parameter. The TTL is a packet parameter that decreases each time a router processes the packet. When the TTL reaches zero, a router discards or drops the packet without forwarding it. A packet caught in a routing loop is removed from the internetwork when its TTL expires.

A limit is set on the number of hops to prevent infinite loops.

Defining a Maximum

Purpose: This figure describes a corrective measure that attempts to solve the routing loop problems that a distance vector protocol could face.

Emphasize: Routing loops occur only when routing knowledge being propagated has not yet reached the entire internetworkwhen the internetwork has not converged after a change. Fast convergence minimizes the chance for a routing loop to occur, but even the smallest interval leaves the possibility open.

To avoid prolonging the count-to-infinity time span, distance vector protocols define infinity as some maximum number. This number refers to a routing metric, such as a hop count.

With this approach, the routing protocol permits the routing loop until the metric exceeds its maximum allowed value. This example shows this defined maximum as 16 hops. Once the metric value exceeds the maximum, network 10.4.0.0 is considered unreachable.

Packets for network 10.4.0.0 bounce (loop) between Routers B and C.

Routing Loops

Slide 4 of 4

Emphasize: In Layer 4, router A receives the new routing table from router B, detects the modified distance vector to network 10.4.0.0, and recalculates its own distance vector to network 10.4.0.0 as 3.

If all routers in an internetwork do not have up-to-date, accurate information about the state of the internetwork, they might use incorrect routing information to make a routing decision.

The use of incorrect information might cause packets to take less-than-optimum paths or paths that return packets to routers that they have already visited.

It is never useful to send information about a route back in the direction from which the original information came.

Split Horizon

Purpose: This figure introduces the corrective measure known as split horizon. The split horizon technique attempts to solve routing loops.

Emphasize: The split horizon technique attempts to eliminate routing loops and speed up convergence. The rule of split horizon is that it is never useful to send information about a route back in the direction from which the original packet came. In the example:

Router C originally announced a route to network 10.4.0.0 to router B. It makes no sense for router B to announce to router C that router B has access to network 10.4.0.0 through router C.

Given that router B passed the announcement of its route to network 10.4.0.0 to router A, it makes no sense for router A to announce its distance from network 10.4.0.0 to router B.

Because router B has no alternative path to network 10.4.0.0, router B concludes that network 10.4.0.0 is inaccessible.

In its basic form, the split-horizon technique simply omits from the message any information about destinations routed on the link. This strategy relies either on routes never being announced or on old announcements fading away through a timeout mechanism.

Split horizon also improves performance by eliminating unnecessary routing updates. Under normal circumstances, sending routing information back to the source of the information is unnecessary.

Routers advertise the distance of routes that have gone down to infinity.

Route Poisoning

Purpose: This figure expands on the split-horizon technique by adding the concept of poisonous reverse updates.

Emphasize: Route poisoning closes the potential for longer routing loops. Fast convergence minimizes the chance for a routing loop to occur, but even the smallest interval leaves the possibility open. With a poison route in place, router B can maintain a steadfast entry that network 10.4.0.0 is indeed down.

Poison reverse overrides split horizon.

Poison Reverse

Purpose: This figure explains poison reverse.

Emphasize: Poison reverse overrides the split-horizon solution.

The router keeps an entry for the possibly down state in the network, allowing time for other routers to recompute for this topology change.

Holddown Timers

Purpose: This figure describes how hold-down timers avoid the general problems that a routing protocol could face.

Emphasize: Hold-down timers are used to prevent regular update messages from inappropriately reinstating a route that may have gone bad.

Hold-downs tell routers to hold any changes that might affect routes for some period of time.

The hold-down period is usually calculated to be just greater than the period of time necessary to update the entire network with a routing change.

Note: A network administrator can configure the hold-down timers on several routers to work together in tandem.

The router sends updates when a change in its routing table occurs.

Triggered Updates

Purpose: This figure describes how triggered updates avoid the general problems that a routing protocol could face.

Emphasize: Normally, new routing tables are sent to neighboring routers on a regular basis. A triggered update is a new routing table that is sent immediately, in response to some change.

Each update triggers a routing table change in the adjacent routers, which, in turn, generate triggered updates notifying their adjacent neighbors of the change. This wave propagates throughout that portion of the network where routes went through the link.

Triggered updates would be sufficient if we could guarantee that the wave of updates reached every appropriate router immediately. However, there are two problems:

Packets containing the update message can be dropped or corrupted by some link in the network.

The triggered updates do not happen instantaneously. It is possible that a router that has not yet received the triggered update will issue a regular update at just the wrong time, causing the bad route to be reinserted in a neighbor that had already received the triggered update.

Coupling triggered updates with holddowns is designed to get around these problems. Because the hold-down rule says that when a route is removed, no new route will be accepted for the same destination for some period of time, the triggered update has time to propagate throughout the network.

Eliminating Routing Loops

Slide 1 of 6

Purpose: This page begins a series of graphics that tie all the solutions together by showing how each solution works to prevent routing loops in a more complex network design.

Emphasize: Begin this series by describing that router B poisons its route to network 10.4.0.0.

Eliminating Routing Loops (Cont.)

Slide 2 of 6

Purpose: This figure continues to describe how the solution works to prevent routing loops in a more complex network design.

Emphasize: Describe that routers D and A both set their hold-down timers and send triggered updates to router E about the status of network 10.4.0.0. Router E also sets its hold-down timer.

Eliminating Routing Loops (Cont.)

Slide 3 of 6

Purpose: This figure continues to describe how the solution works to prevent routing loops in a more complex network design.

Emphasize: Router A and D send a poison reverse to router B. Router E sends a poison reverse to router B.

Eliminating Routing Loops (Cont.)

Slide 4 of 6

Purpose: This figure continues to describe how the solution works to prevent routing loops in a more complex network design.

Emphasize: While in hold-down state, routers A, D, and E will still attempt to forward packets to network 10.4.0.0.

Eliminating Routing Loops (Cont.)

Slide 5 of 6

Purpose: This figure continues to describe how the solution works to prevent routing loops in a more complex network design.

Emphasize: When the link is back up, router B will send a triggered update to router A and router D, notifying them that network 10.4.0.0 is active.

Eliminating Routing Loops (Cont.)

Slide 6 of 6

Purpose: This graphic continues to describe how the solution works to prevent routing loops in a more complex network design.

Emphasize: Router A and D, in turn, update router E that network 10.4.0.0 is now up.

Summary

Distance vector routing protocols generate periodic routing updates addressed to directly connected routing devices. Routers running a distance vector routing protocol send periodic updates even if there are no changes in the network.When a router receives an update from a neighbors router, the router compares the update with its own routing table. The router adds the cost of reaching the neighbors router to the path cost reported by the neighbor to establish a new metric.Routing inconsistencies occur if slow internetwork convergence or a new configuration causes incorrect routing entries.

Purpose: This slide discuss the initial configurations on the routers and switches.

Note: There is no setup mode on the Catalyst 1900 switch.

Summary (Cont.)

Distance vector protocols define infinity as some maximum number. The routing protocol then permits the routing table update loop until the metric exceeds its maximum allowed value.There are five techniques for eliminating routing loops on distance vector routing networks: split horizon, route poisoning, poison reverse, holddown timers, and triggered updates.All five techniques can be used together to eliminate routing loops in area networks.

Purpose: This slide discuss the initial configurations on the routers and switches.

Note: There is no setup mode on the Catalyst 1900 switch.